Prior vehicle position determining systems are known in which sensors on the vehicle are utilized to determine vehicle position and heading based on sensed distance traveled and sensed changes in vehicle heading. In such systems, which are generally designated as dead reckoning position systems, the initial vehicle position is determined either by the vehicle operator directly entering such information into a computer, or through the utilization of a Loran or GPS (global positioning system) vehicle location system or by using the vehicle's previously calculated position. The initial vehicle heading is determined by either an electronic or magnetic compass or by using the vehicle's previously calculated heading. Then, the distance traveled by the vehicle and the change in vehicle heading are measured by vehicle mounted sensors and the new vehicle position is determined by measuring these changes over a predetermined time or distance interval, effectively creating a distance and heading change vector and adding this change vector to the initial vehicle position and heading to arrive at a new vehicle position and heading. In some prior systems, the new position calculated by these systems is then compared to map data stored in a computer and the vehicle position is corrected since the vehicle is assumed to only be able to have a location corresponding to the location of roads defined by the stored map data. The operation of systems of the above type is well known.
In some of the systems described above, the sensed distance which the vehicle travels is determined by wheel sensors which measure the wheel speed or revolutions of the wheel. Sometimes, two wheel sensors are utilized and the average of these wheel sensors is utilized to determined vehicle distance traveled. In addition, the difference between wheel revolutions or wheel speed can be utilized to determine changes in vehicle heading. Changes in magnetic compass readings may also be utilized, along with such two wheel sensors, to determine changes in vehicle heading. Systems which operate in accordance with the above stated principals are subject to errors which may not be readily correctable just by comparing the calculated new vehicle position and heading with allowable vehicle positions and headings based on stored roadway map data. Such errors in calculated vehicle position and heading can occur, for example, when during vehicle travel a bump in the road is encountered. This bump may cause a substantial difference between the revolutions of the two wheels which are sensed to determine distance traveled and changes in vehicle heading. Applying the brakes of the vehicle when the vehicle goes over a bump can magnify this difference error, especially if one of the sensed wheels becomes airborne whereas the other sensed wheel does not.
Some prior navigation systems and/or vehicle position determining systems discuss providing a signal indicative of actual sensed lateral acceleration of the vehicle. This lateral acceleration is then utilized, in combination with sensed vehicle speed, to determine the distance and heading change vector to be utilized in determining vehicle position. However, the lateral acceleration is not utilized to correct for erroneous sensor readings, and in fact appears to be subject to such erroneous sensor readings. In some navigation systems, such as for guided missiles, the route to be selected for the vehicle may take into account the maximum lateral acceleration or operating capabilities of the vehicle. However, clearly this has nothing to do with correcting errors in a vehicle position determining apparatus which calculates a vehicle new position based on its old position and heading and based on changes in distance traveled and sensed changes in vehicle heading.